This invention relates to a process that provides multi-step methods of extracting and purifying naturally occurring zeolite from zeolitic ores in the presence of other mineral phases having various properties. More specifically, this invention relates to a semi-continuous method for obtaining a highly enhanced, low bulk density zeolite product which displays increased zeolite concentration, improved brightness, elevated ion exchange capacity, and enhanced rheological properties. The present invention is practiced without the use of polymeric flocculating, dispersing, or floatation materials which result in contamination of the resultant zeolite, and includes a separate step for classifying the zeolite by particle size and mineral phase.
Natural zeolites are hydrated aluminosilicates of alkali and alkaline earth metals. Zeolites have a crystalline structure commonly known as framework aluminosilicates with infinitely extending three dimensional networks of AlO4 and SiO4 tetrahedra linked to each other by the sharing of all oxygens. This three dimensional network structure provides extensive surface area within the zeolite, with up to 50% of zeolite volume attributable to the channels and cavities. This property contributes to a specific gravity that is intermediate to other mineral phases of the natural ore, which increases the difficulty of separations by prior art methods.
Natural zeolites are used in a variety of applications, including, for example, ion exchange, radioactive waste treatment, industrial waste treatment, uses as animal feed supplements, moisture absorbents, carriers for time-released substances such as pesticides or fertilizers, liquid and gas filters for contaminant and odor control, oil absorbents, and industrial coatings and fillers. Naturally occurring zeolites are also frequently used as starting material in synthetic zeolite production. Zeolites have also demonstrated usefulness as catalysts in hydrocarbon conversion reactions. The large surface area of zeolite makes it an excellent choice for such applications.
Another feature of zeolite structure is that the cavities within a particular zeolite are all of uniform shape and size. Consequently, natural zeolites may act as analogs to artificial molecular sieves.
One natural zeolite, clinoptilolite, possesses a particularly high absorbing capacity due to its large surface area. Furthermore, clinoptilolite offers a high cation exchange capacity, making it suitable for use in numerous industrial applications. The pore size of clinoptilolite makes this zeolite well suited to waste water filtration, particularly due to its demonstrated selectivity for strategic ions. Finally, the thermal and physical stability and compatibility with final waste forms, such as cement or glass, make it an attractive alternative to less stable and incompatible options such as polymer based ion exchange resins.
Naturally occurring zeolite ores are well known to contain a variety of contaminants, including, for example, clay, quartz, mica, feldspar, iron and titanium minerals and calcites. Naturally occurring zeolites have heretofore been effectively excluded from certain applications which require extreme brightness, such as in the fine paper industry where higher cost titanium dioxide, calcium carbonate or silica may be preferred additives. Furthermore, naturally-occurring zeolites are frequently passed over for use as molecular sieves or as catalysts because of ineffective purification methods.
Currently available and prior art process for extraction, purification and classification of natural zeolites are very limited and not commonly practiced. The art of clay minerals benificiation having been applied to zeolite ores includes pulverizing, dry classifying or wet gravitational separations, magnetic separation, bleaching and calcining to drive water out of the pores has proven relatively ineffective. A number of such processing techniques have been described in the prior art for zeolite applications, but have not been commercially successful. For example, in U.S. Pat. No. 4,510,254, a batch process is described in which a zeolitic ore is processed through the steps of pulverization, slurrying, removal of fines, fine milling, magnetic separation, bleaching and drying. The ""254 process results in a dry finely ground zeolite having a particle size of below 2 microns and a TAPPI brightness of at least 90. Zeolite obtained from the ""254 process also possesses a bulk density of about half or less of high quality kaolin clay pigments. Despite the claimed properties of zeolite obtained from the ""254 process, natural zeolite deposits remain difficult to treat to sufficient purity, brightness, size discrimination and density. The overall yield of ""254 process is less than 20% compared to greater than 40% for the present invention. Similarly, the zeolite content of the product obtained from the ""254 process is slightly lower than the mineral source whereas the process of the present invention enhances the zeolite content by over 20%. Processes according to the prior art, including the ""254 patent, typically include complex chemical methods that introduce undesirable chemical contamination as part of the purification process and generate large waste inventories having difficult and costly treatment requirements. Introduction of chemical contaminants further complicates the prior art processes by requiring intermediate steps to ameliorate the affect of such chemical contaminants. Some prior art classification techniques, such as that described in U.S. Pat. No. 5,603,411, also add undesirable chemical contaminants such as flocculating agents and dispersants. Consequently, artificially produced zeolites and polymeric resins remain the prime choice in high-end applications, such as ion exchange. Similarly, high cost minerals such as titanium dioxide and calcium carbonate are preferred over zeolite in fine paper manufacturing.
The absence of natural zeolite competition in various high end applications is largely an issue of effective and economic processes for extraction and purification from zeolite ores. There remains a need therefore, for a process which permits economical and technically sufficient exploitation of naturally occurring zeolite materials for a broad variety of applications. Furthermore, there is a need for a process which does not introduce chemical contaminants, such as flocculating or dispersing agents, and which does not produce hazardous process effluents.
The present invention provides an efficient, cost-effective process for the extraction and purification of natural zeolite, from mined ores which include clinoptilolite and mordenite, by a novel method of separation from other mineral phases present in mined ores. A process has been discovered that enables effective separation of mineral phases by mechanical dispersal and differential suspension of respective minerals according to both their physical and chemical properties. The novel process exploits the properties of demineralized water to facilitate differential suspension of fine particles without the use of dispersants or other chemical suspending agents and does not include chemical flocculating agents or floatation agents that are all common to the prior art.
The process of the present invention recognizes and exploits variation in fundamental properties of respective mineral phases including extent of hydration and influence on effective volume of particles to promote differential suspension. Demineralized water facilitates maximum electrical double layer repulsion between particles and minimizes the influence of electrolytes on the electrical double layer to preclude flocculation.
The process of the present invention first hydrates and mechanically disperses the starting material to separate out the highly hydrated clay content followed by separation of the zeolite from contaminants having a higher mass to surface area ratio than that of the desired zeolite by use of one or more countercurrent flow separation columns in which the dispersing medium is demineralized water.
The resulting purified product displays an increased zeolite concentration, improved brightness, and elevated ion exchange capacity. Further treatment with magnetic separation and fine grinding provides a high brightness and low bulk density product suitable for fine paper coatings and fillers in industrial whitening applications. The rheological properties are enhanced by removal of clay and high-density minerals. Since the residual contaminants are often near detection limits of the analytical method, the efficacy and mass balance are best confirmed in the tailings of the process where they are more concentrated. Existing chemical leaching techniques may further enhance the zeolite product. However the excellent properties from the novel process may preclude the necessity of this expensive step for many applications while raising the process yield to about 59%.
The present invention further provides a new method of wet classification of the purified zeolite stream. The wet classification system of the present invention provides a significant and substantial improvement over prior art methods of classifying fine zeolites, primarily by density difference, and also employs the properties of demineralized water and the electrical double layer of hydrated mineral phases to amplify their differences in settling velocity. The wet classification method of the present invention may be applied to other types of fines, such as in the processing of kaolin clays, finely ground mineral ores and synthetically produced zeolites.
Overall, the present invention is much less complex than the prior art and substantially contributes to cost-effectiveness through higher yields, lower capital cost and reduction of waste liabilities.